(a) Field of the Invention
The present invention relates to a retroreflective article and, more particularly, to a retroreflective article which exhibits improved performance characteristics in retroreflective capacity and angularity.
(b) Description of the Related Art
Retroreflective articles are widely used for various purposes. For example, they are employed for use in decorations, reflection tapes for safety, reflection belts, road signs, and reflection plates for warning. Retroreflective article is based on a light retroreflection principle that is redirecting light toward opposite direction to incident direction, such a retroreflection is occurred by total internal reflection of light from three lateral sides which are mutually perpendicular in cube corner.
Generally, the retroreflective article is formed with optically transparent materials such as glass or polymethyl metacrylate (PMMA) and is sheet-shaped with one surface processed to have continuous arrays of cube corners on the surface.
The term “cube corner” can be easily understood by supposing triangular pyramid deriving from cutting one corner portion of a cube. Fundamental structure of a cube corner is a triangular pyramid having a base side of triangle and three lateral sides being perpendicular to each other, and fundamental structure of one surface or retroreflective article consists of continuous arrays of such a triangular pyramid. In particular, plan view of retroreflective article being processed to that cube-corners being types of regular triangular pyramids having regular triangle as a base planes arrange continuously on its one surface, is shown in FIG. 1.
An optical axis of the cube corner is defined as a line drawn through the apex to the base while keeping the same angular relation to the three lateral sides. A front axis direction is defined as a line extended normal to the retroreflective article sheet. The light falling on the retroreflective article in the front axis direction is well retro-reflected, whereas the light falling on the retroreflective article while deviating from the front axis direction by a predetermined angle exhibits poor retroreflective performance because it does not satisfy the necessary conditions for total internal reflection from the three lateral sides of the triangular pyramid.
The term “angularity” is commonly used to describe the retroreflective performance characteristic of the retroreflective article with respect to light falling on the retroreflective article deviating from the front axis direction by a predetermined angle. It is preferable that the retroreflective article exhibits wider angularity. Practically, it is legally regulated that the angularity of the retroreflective article for road sing should reach a predetermined degree at the front axis direction and light incidence angle of 30°.
The retroreflective article with the regular triangular pyramids(1) as shown in FIG. 1 exhibits a very narrow angularity. FIG. 2 is a graph illustrating the results of measuring the retroreflection capacity of the retroreflective article shown in FIG. 1 as a function of the light incidence angle of 0°˜40° to the front axis direction. The graph of FIG. 2 is indicated in percentage in which maximum retroreflection capacity is obtained in case that lights are incident in front axis direction. As shown in FIG. 2, the fundamental retroreflective article consisting of the cube-corners being types of regular triangular pyramids exhibits roughly uniform retroreflective capacity in the light incidence angle of 0°˜20° to the front axis, and has no relation with direction of incidence angle, desirably. But the retroreflective capacities rapidly drop in the light incidence angle of 20°˜30° or more.
As an attempt to obtain wider angularity, it has been suggested that the shape of the regular triangular pyramids arranged on one surface of the retroreflective article sheet should be appropriately changed. The change is occurred by tilting optical axis of the cube-corner in a predetermined direction, so that the regular triangle of the base is transformed to an isosceles triangle or unequilateral triangle. Tilting optical axis of cube-corner to predetermined direction produces that retroreflection capacity deteriorates in the front axis direction, but enhances in the predetermined direction.
U.S. Pat. No. 4,588,258 discloses a method obtaining improved retroreflection capacity by tilting optical axis of the cube-corner having regular triangular pyramid structure. FIG. 3 is a plane view of retroreflective article based on U.S. Pat. No. 4,588,258 where the optical axis of the cube corner is tilted by about 7°-13° to the front axis, and the resulting retroreflective article exhibits enhanced retroreflective capacity with respect to the light entering with the incidence angle of 30=20 or more in the X and Y axis directions which are mutually perpendicular and are on the retroreflective sheet plane.
FIG. 4a and FIG. 4b which are shown in order to indicate inclination of optical axis of cube-corner forming retroreflective article as shown in FIG. 3, are cross sectional view taken along the 4A and 4B line in FIG. 3, respectively. As shown in FIG. 4a and FIG. 4b, optical axis(x) has tilting angle to front direction(f) of retroreflective sheet.
FIG. 5 illustrates the results of computer-simulating the retroreflective performance characteristics of the retroreflective article consisting of the cube-corners having such a optical axis tilting. FIG. 5 is isobrightness curves indicating equal retroreflection coefficient value in case of introducing lights in several direction to retroreflective article. As shown in FIG. 5, when the light incidence angle varies from the front axis to the Y axis, suitable degree of retroreflective capacity is obtained with a light incidence angle of 40° or more. Furthermore, even when the light incidence angle varies from the front axis to the X axis, relatively better retroreflective capacity is obtained but less than that in the Y axis direction. That is, relatively wider angularity can be obtained with respect to the X and Y axis directions. However, very narrow angularity is shown in a K axis direction which is on a middle position between the X and Y axes.
FIG. 6 shows graphs indicating results of retroreflection capacity measurements varying incident angle with respect to each two case, of which one corresponds that cube-corners included in one plane of retroreflective sheet are regular triangular pyramids and the other corresponds that cube-corners are transformed from the regular triangular pyramids to have 9.2° tilting from front axis of retroreflective sheet. When the cube-corner is regular triangular pyramid, the retroreflective capacity measurement is performed by varying incidence angle with respect to the X and Y axis directions. In contrast, when the optical axis of the cube-corner is tilted by 9.2°, the measurement is performed by varying incidence angle with respect to the X, Y and K axis directions. As described above, when the optical axis of the cube-corner is tilted, retroreflection capacity exhibits wider angularity even though lower in near direction by the front axis with respect to the X and Y axis directions, but vary narrow angularity in the K axis direction.
There is a method of tilting the optical axis by negative angle differently from the method of tilting the optical axis by 7-13° in U.S. Pat. No. 4,588,258. U.S. Pat. No. 2,310,790 discloses a retroreflective article where the angularity with respect to the light entering normal to the plane including the optical axis, in enhanced by tilting the optical axis of the cube-corner by −7.4.
However, with such cube corners having tilted optical axes, the retroreflective capacity may be enhanced only in the predetermined light incidence direction but not in other various light incidence directions. In order to solve such a problem, a tiling technique is suggested. In the tiling technique, retroreflective articles exhibiting good retroreflection capacity in a incidence angle are tilted directionally, so that retroreflection capacity is expected to be enhanced with respect to various light incidence angle. However, with the application of the tiling technique, the total brightness drops.
In the above-described techniques, since the tilting angle of the optical axis varies in upper and lower directions, the base of the cube corner is shaped with an isosceles triangle. Alternatively, the tilting angle of the optical axis varies in right and left directions as well as upper and lower directions such that the base of the resulting cube corner is shaped with an unequilateral triangle. This is to enhance the retroreflective capacity with respect to various light incidence directions. For instance, U.S. Pat. No. 5,822,121 discloses a retroreflective article where the base of the cube corner is shaped with an unequilateral triangle while the tilting angle of the optical axis is controlled by 4-15°. However, such an article involves complicated processing steps because three forms of bits should be used for processing grooves of the cube corner. Furthermore, with the cube corner having an unequilateral triangle-shaped base, it is difficult to obtain good retroreflective capacity at increased light incidence angle.
Alternatively, a retroreflective article based on sets of different sized cube corners may be provided. U.S. Pat. No. 5,840,406 discloses a retroreflective article where different sized cube corners are arranged at the same time by removing some parts of the cube corners such that internal light can be allowed to transmit through naturally generated surface at relatively small size cube corner. U.S. Pat. No. 5,122,902 also discloses a cube corner having a flat or curved cutting plane for allowing transmission of the light. However, in these techniques, the total brightness of the retroreflective article drops because the retroreflective article is structured such that the incident light is partially transmitted.